Shock strut with linear damper



Feb. 26, 1963 J. W. BROWN ETAL SHOCK STRUT WITH LINEAR DAMPER File d Aug. 15, 1960 4 i v Z INVENTOR. JA/Vtj 14/. 590W dyh'iiifid? Patented Feb. 2%, 1%53 3,075,967 SEQ-CK STRUT WHTH LINEAR DAMEER James W. Brown and Raymond 3. Black, South Bend, ind, assignors to The Bendix Corporation, South Bend, Ind, a corporation of Delaware Filed Aug. 15, 1966, Ser. No. 4%,566 4 Claims. (Cl. 183-96) The present invention relates to a shock stru-t particularly adapted for use in a helicopter landing gear.

One of the problems facing the helicopter landing gear designer is that of ground resonance. Ground resonance is a phenomenon which can occur during either landing or take-off operation. It occurs when a slightly out-ofbalance condition of the spinning rotor blade induces a pitching and rolling motion to the airframe on its wheels. Rotor unbalance can be aggravated by the blade drag hinges used on most rotor assemblies. Drag hinges are the couplings between the blades and the rotor shaft. At low speeds, the drag hinges can permit unequal spacing of the blades thus producing the out-of-balance condi tions.

The kinetic energy of the pitching or rolling mass of the airframe is transferred to the landing gear. When this energy is transferred at a rate greater than the energy dissipating rate of the landing gear, a self-excited resonance can occur. There have been instances of catastrophic failure of major components of helicopters resulting from loadings imposed under these ground resonance conditions. Accordingly, it is an object of the present invention to provide a landing gear shock strut for helicopters having linear damper means for reducing ground resonance oscillation.

Another object of the present invention is to provide in combination with a shocl: strut of the variable orifice type and a metering pin for controlling the orifice area, a damping piston secured to the metering pin for damping low amplitude vibrations.

Other objects and features of the invention will become apparent from a consideration of the following description which proceeds with reference to the accompanying drawings, wherein:

FlGURE 1 is a section view of a shock strut in accordance with the present invention, and;

FIGURE 2 is an enlarged section view of a portion of PEGURE 1 providing a clearer il ustration of the linear damper portion of our shock strut.

Referring to FIGURE 1, a shock strut, incorporating the novel principles of the present invention, is generally indicated by reference numeral ill, and includes an outer tubular casing 12 which normally is secured to an airframe by means of a bracket partially shown at 14. A second tubular member 16 contained concentrically within casing 12 is threadedly secured to a retainer 18 at its upper end which in turn is secured to the upper end of casing 12, by means of nut such that the tubular members 12 and are relatively fixed. The outer surface of tubular member 16 and the inner surface of tubular casing 12 define a hydraulic fluid containing annular cham er 22 which is further separated into two compartments 2% and 26 by a movable tubular member 2:} which is telescoped between said fixed tubular members. An annular bushing as is threadedly secured to the upper end of movable tubular member 2.8 to provide sliding bearing support with the inner surface of casing 12. Ports 2% are provided in the side wall of tubular member 28 near bushing 3% to provide intercommunication between compartments 24 and 26. The lower end of casing 12 includes a pair of annular bushings 32 inserted in a stepped diameter portion and held in position by threaded cap 34-. Bushings 32 include annular slots for receiving fluid seals 36. The second fixed tubular member 16 carries a diaphragm A nut 43 is screwed onto the threaded end portion of tubular member 16 to clamp the diaphragm 38 into place. A metering orifice 42 is formed in the diaphragm 33 and a metering pin 44, which is operatively carried by movable tubular member 23 through the removable end plate 46, is movable through the metering orifice 42; to control the size of the orifice which is available for metering fluid therethrough. Nut as contains a fluid seal 48 mounted thereon which provides a fluid tight compartment 5% formed between diaphragm 38 and end plate 45. All fluid entering or leaving compartment 56 must pass through orifice 42. Fluid forced from chamber 5t} passes through orifice 42 and through the additional openings 5'2 provided in the sidewall of tubular member 16 into chamber 22.

When the strut is fully compressed such as illustrated in PEGURE 1, the liquid level of the hydraulic fluid rises to the level indicated by the legend, approximately up to annular bushing 3d. The remaining volume contains a compressible fluid medium such as air or the like.

The parts and structure described up to this point comprise a conventional aircraft shock strut of the variable metering orifice type primarily adapted for absorbing impact energy during aircraft landings. As the aircraft containing such a strut approaches a landing, the movable tubular member 28 is fully extended providing a maximum volume in compartment 53% As the aircraft wheels, which are secured in any well-known manner to the lower end of movable member 28, touch down and the aircraft load is transferred from the wings, rotor or other load supporting airfoil to the landing gear, tubular member 23 elescopes within members 12 and id. The telescoping action forces fluid from compartment St} to chamber 22 through orifice 42 at a rate permitted by metering pin 44 which may have a variable area along the portion of its length remaining below Wall member 7 to achieve the most cfiicient load absorbing characteristic. As fiuid is forced into and occupies the volume in chamber 22 the compressible fluid or air contained in said chamber is compressed. As the aircraft comes to rest and all its weight is supported by the landing gear, the full aircraft wci ht is in effect riding on a compressed column of air. Such action provides a highly efiicient aircraft landing energy absorbing means, however, small cyclic vibrations or variations are not effectively countered since such a strut is primarily designed to meet large stroke high energy absorption conditions. The previously described phenomenon of ground resonance may induce cyclic vibrations which build up to a high magnitude in a shock strut designed to meet the landing conditions pointed out above. Accordingly, our invention comprises the util zation of linear damper means in combination with the above described variable metering orifice shock absorber to provide a shock strut that is not only capable of effisprees? 3 ciently absorbing landing energy, but in addition effectively and efiiciently dampens out small amplitude vibrations induced by conditions such as ground resonance. The structure comprising the resonance damper will now be described.

A hollow cylinder 53 comprised of two threadedly joined tubular sections 64} and 62 is positioned within the second fixed tubular member 16 at a position determined by spacing sleeve 6 inserted between the lower end of cylinder 58 and diaphragm 38. Cylinder 5t} abuts at its upper end against retainer 18. At the threaded juncture of the tubular sections 69 and 62 2. Wall member 65 is retained and includes a small calibrated orifice 6S and two spring loaded check valves 75) and 72 which permit flow in opposite directions when the check valve spring loading force is overcome. A second wall member 74 is secured to the lower end of cylinder 62 and with the side Walls of said cylinder and wall member 66 forms a chamber 76 which contains a column of hydraulic fluid. Wall member '74 has an opening 75 therein for receiving metering pin 44in a fluid tight manner. A piston 73 is attached to the upper end of metering pin 44 and includes a fi'cd small bore calibrated passage 8i} and a second 'ar ,e bore relief passage 32. The piston 78 operates in the column of fluid in chamber 76 and the damping force, required under short stroke high frequency cycling of the shock strut is provided by passing fluid from one side of piston 72'; to the other through restrictive passage The difiference in fluid volume in chamber 76 due to the varying displacement of metering pin 44 is compensated for by allowing excess fluid from chamber 76, above piston 8%, to pass through orifice 68 in wall as to a compensating chamber 84 formed in the upper tubular section it A movable piston or wall member 36 is provided in tubular section st} to provide a separation barrier between the hydraulic fiuid in chambers 34 and '76 and the compressible lluid or air in the uppermost section of the shock absorber. Hydraulic fluid entering chamber 54 thus force-s piston 36 upwardly to compensate for the variation in fluid displacement in chamber 76 due to the entrance therein oi metering pin 44. During the extension portion of the shock absorber cycle, the reverse action takes place and the excess fluid required is returned from chamber 3d to chamber 76 through orifice 63 by action of the air pressure on the top of floating piston 35. if desired, a spring member may also be employed to urge piston as downwardly to insure more positive positioning and reduce the probability of cavitation in chambers 76 and Check valve it! is provided in Wall member 66 to further reduce the probability of cavitation and to permit rapid transfer of hydraulic fluid during long strolte rapid movements of metering pin 44.

A movable disc member dd is slidably mounted on the upper end of metering pin 44 immediately below piston 66 and includes a valve member 92 secured thereto arranged to close large diameter passage Disc 3% and valve 92 are urged upwardly in a direction to close passage 82 by force producing spring 94 concentrically mounted around metering pin 4d held in position by a tired disc member 95 also secured to metering pin 44.

During normal long stroking of the shock absorber such as occurred in the previously described aircraft landing, sufficient pressure is built up in chamber 76, between piston 78 and Wal 6:3, to unseat valve 92 against the action or" spring 94, thus permitting rapid transfer of fluid in hamber 76 from above to below piston 73. The pressure built up between piston '78 and wall 66 is also high enough to unseat check valve 72 to allow for rapid compensation for displacement of metering pin 44. Shock absorbing characteristics under these conditions are controlled by metering pin 44 and orifice 42 configurations and are not materially attested by the resonance damper. After the aircraft has come to rest and piston 78 is not traversing chamber 76 rapidly, valve 532 closes, and piston 78 becomes operative as a damping agent, as only the relatively small restrictive passage 84} is operative to pass fiuid to opposite sides of piston 78. During extension of the shock strut, for example after take off, fluid enters the chamber between piston '1" 3 and wall 66 through passages 68 and 80. It has been found unnecessary for aircraft installations to provide means for permitting rapid shock absorber movement in the extending direction, although quite clearly the provision of such means is merely carrying forward the ideas disclosed herein and would be within the ambit of the ordinary mechanic.

Although only one embodiment of our invention has been disclosed, it should be understood that this is merely the preferred embodiment, and that other arrangements employing the novel aspects defined in the claims are within the scope of this invention.

We claim:

1. A shock strut comprising first and second relatively movable telescoping elements defining a variable volume fluid containing chamber, an orifice plate having a fluid control orifice formed therein movable with said first telescoping element and forming one side wall of said variable volume chamber, a metering pin contoured over a portion of its length secured to said second telescoping element and movable therewith, said metering pin arranged to extend through said fluid control orifice of said orifice plate to define a variable effective area control orifice, a cylinder member movable with said first telescoping element containing a confined column of hydraulic fluid 'coaxially arranged with said metering pin, said cylinder member having a fluid tight opening at one end for receiving said meteringpin while preventing liuid transfer during the full operative range of shock strut movement, and a piston member secured to said metering pin within said cylinder member for movement therewith, said piston member having a relatively small diameter restrictive passage formed therethrough to permit fluid transfer from opposite sides of said piston with said cylinder member at a controlled rate.

2. A shock strut as claimed in claim 1 wherein said piston member includes a second relatively large diameter passage formed there-through in parallel flow relationship with said relatively small diameter restricted passage to permit relatively unrestricted fluid transfer from opposite sides of said piston, pressure responsive spring loaded valve means for normally closing said large diameter passage, said valve means further operative to open said relatively large diameter passage under the influence of a hydraulic fluid pressure dilierential of a predetermined minimum value during the compression stroke of said telescoping elements.

3. A shock strut of the hydraulic-pneumatic type comprising first and second relatively movable telescoping elements defining a variable volume iluid containing chamber, fiuid control means defining an orifice communicating with said variable volume chamber movable with said first telescoping element, an elongated metering pin extending through said orifice for controlling the slicetive flow controlling area thereof movable with said second telescoping element, a hollow cylindrical member containing a column of hydraulic fluid arranged within movable with said first telescoping element in coalignment with said metering pin, said cylindrical member including a first fixed wall member and a second fixed wall member disposed in said cylinder to define a fired volume fluid containing compartment in cooperation therebctween said first fixed Wall member having an opening formed therein for receiving said metering pin therethrough so that one end of said metering pin is re ciprocable in said fixed volume compartment, and a piston member secured to said metering pin within said fixed volume compartment, said piston member including a first restrictive fluid permitting passage formed therethrough 3,978,967 5 \5 for damping the movement of said piston and metering References Cited in the file of: this patent pin members.

4. A shock strut as claimed in claim 3 wherein said UNITED STATES PATENTS piston member includes a relatively large relief passage 2,171,827 Elliott Sept 5, 1939 formed therein, valve means operative with said relief 5 Q passage to control the hydraulic fluid flow therethrough, g gi g g spring means operative to bias said valve means in a pas- A run 16 J J sage closing position, said valve means being operative 2,77446 Carbon 1956 to open against the action of said spring means when the 2,315,099 Muller et a1 19 pressure difierential across said piston exceeds a prede- 10 2,886,142 OrShansky May 12, 1959 termined value. 

1. A SHOCK STRUT COMPRISING FIRST AND SECOND RELATIVELY MOVABLE TELESCOPING ELEMENTS DEFINING A VARIABLE VOLUME FLUID CONTAINING CHAMBER, AN ORIFICE PLATE HAVING A FLUID CONTROL ORIFICE FORMED THEREIN MOVABLE WITH SAID FIRST TELESCOPING ELEMENT AND FORMING ONE SIDE WALL OF SAID VARIABLE VOLUME CHAMBER, A METERING PIN CONTOURED OVER A PORTION OF ITS LENGTH SECURED TO SAID SECOND TELESCOPING ELEMENT AND MOVABLE THEREWITH, SAID METERING PIN ARRANGED TO EXTEND THROUGH SAID FLUID CONTROL ORIFICE OF SAID ORIFICE PLATE TO DEFINE A VARIABLE EFFECTIVE AREA CONTROL ORIFICE, A CYLINDER MEMBER MOVABLE WITH SAID FIRST TELESCOPING ELEMENT CONTAINING A CONFINED COLUMN OF HYDRAU- 